1. Field of the Invention
The present invention relates to a vehicle driveline clutch actuator and more specifically to a vehicle driveline clutch actuator using a ball ramp mechanism having a control ring attached to a pivoted weight acting on a friction element.
2. Description of the Prior Art
Driveline clutches commonly use a plurality of springs to clamp a friction disc to an engine flywheel. The springs are disposed within a pressure plate assembly which is bolted to the flywheel. A mechanical linkage that controls the pressure plate spring mechanism is displaced by the operator to control the lock-up and release of the clutch.
Efforts to automate the operation of the clutch using electronics are currently underway. It is known to use an electromechanical or hydraulic actuator connected to the mechanical linkage to, in essence, replace the operator for more accurate clutch operation during transmission shifting. Using such an actuator, the mechanical linkage is moved in response to an electrical control signal generated by a central microprocessor used to process a variety of vehicle sensor inputs and other operating conditions to determine when and in what manner the driveline clutch should be activated, or deactivated.
The use of a ball ramp actuator to Icad a clutch pack in a vehicle driveline differential is known. U.S. Pat. Nos. 5,092,825 and 4,805,486, the disclosures of which are hereby incorporated by reference, disclose limited slip differentials where a clutch pack is loaded in response to the activation of a ball ramp actuator initiated by rotation of a servo motor or a solenoid driven brake shoe on an activating ring. The advantage of the ball ramp mechanism over other actuators is that it converts rotary motion into axial motion with a very high force amplification, often 100:1 or greater. A ball ramp actuator has also been utilized in a vehicle transmission to engage and disengage gearsets by loading a gear clutch pack in response to a signal as disclosed in U.S. Pat. No. 5,078,249 the disclosure of which is hereby incorporated by reference.
In both of these applications, one side of the ball ramp actuator, commonly called a control ring, reacts against case ground through the force induced by an electromagnetic field generated by a coil or is rotated by an electric motor relative to case ground. To generate greater clamping forces, the electrical current supplied to the coil or motor is increased thereby increasing the reaction of the control ring to case ground which rotates the control ring relative to an activation ring thereby causing rolling elements to engage ramps in the control and activation ring which increase the axial movement and clamping force on the clutch pack. However, a limitation of the prior art is that the use of additional electrical current to continuously maintain the clamping force is inefficient and requires that the coil be large to accommodate the current without overheating. More importantly, by reacting the control ring to case ground, a large amount of rotation slip is constantly present which results in wasted power.
Another problem with the prior art is that speed sensors are required to measure flywheel speed and transmission input shaft speed and a microprocessor is required to rapidly interpret these speed signals, calculate a slip speed and then generate a control signal to the coil to increase the axial lock-up force of the ball ramp actuator should slip occur through the clutch. This approach is slow and excessive slip occurs before action can be taken to increase the clamping force. Excessive slip wears out the clutch prematurely.
Another problem is with vehicle start-up when the clutch is first engaged under a variety of operating conditions. The engagement using the prior art system can be rough and unpredictable with excessive clutch slip or abrupt engagement. This operation results in premature wear, driveline shock and driver complaints.